1. Field of the Invention
The present invention is generally in the field of telecommunications devices and circuits. More specifically, the invention is in the field of telephone line interface circuits.
2. Background Art
A hookswitch relay (also referred to as a xe2x80x9crelayxe2x80x9d in the present application) determines whether a telephone device is xe2x80x9coff-hookxe2x80x9d or xe2x80x9con-hookxe2x80x9d. This determination is made by allowing or disallowing DC telephone line current to flow through the telephone line interface circuit. Off-hook describes the condition when DC line current is allowed to flow through a telephone line interface circuit which can be coupled to a communications device such as a modem. On-hook describes the condition when DC line current is not allowed to flow through the telephone line interface circuit. The use of a hookswitch relay has been thus far generally considered an essential element in a telephone line interface circuit coupled to a communications device such as a modem.
FIG. 1 shows an exemplary telephone line interface circuit 100. System side device 102 (also referred to as SSD 102) is shown in block diagram form. SSD 102 can be part of a communications device such as a modem.
Coupling transformer 106 comprises primary winding 120 and secondary winding 122. One terminal of primary winding 120 is connected to resistor 104 at node 101. The other terminal of primary winding 120 is connected directly to TXA2 of SSD 102. Secondary winding 122 has one terminal connected to capacitor 110. The other terminal of secondary winding 122 is connected to the ring terminal of the telephone line at node 111. Resistor 104 has one terminal connected to a terminal of primary winding 120 at node 101. The other terminal of resistor 104 is connected to TXA1 of SSD 102.
One terminal of capacitor 110 is connected to one terminal of secondary winding 122. The other terminal of capacitor 110 is connected to one AC signal terminal of diode bridge 114 at node 107. One terminal of relay 116 is connected to one AC signal terminal of diode bridge 114 at node 107. The other terminal of relay 116 is connected to the xe2x80x9ctipxe2x80x9d terminal of the telephone line at node 109 (the telephone line is not shown in any of the Figures). In the present discussion, telephone line terminals tip and ring can be interchanged without affecting the operation of the telephone line interface circuit.
One AC signal terminal of diode bridge 114 is connected to relay 116 at node 107. The other AC signal terminal of diode bridge 114 is connected to the xe2x80x9cringxe2x80x9d terminal of the telephone line at node 111. The DC positive terminal of diode bridge 114 (shown as xe2x80x9c+xe2x80x9d) is connected to the DC positive terminal of electronic inductor 112 through line 162. The DC negative terminal of diode bridge 114 (shown as xe2x80x9cxe2x88x92xe2x80x9d) is defined and referred to as DC ground.
The DC positive terminal of electronic inductor 112 is connected to the DC positive terminal of diode bridge 114 through line 162. The DC negative terminal of electronic inductor 112 is connected to DC ground through line 166. MOV 118 has one terminal connected to the tip terminal of the telephone line at node 109. The other terminal of MOV 118 is connected to the ring terminal of the telephone line at node 111.
Coupling transformer 106 provides isolation and impedance matching between SSD 102 and the telephone line. The value of resistor 104 is chosen to set a desired impedance of SSD 102 for properly interfacing with the telephone line. If coupling transformer 106 is assumed to be ideal, i.e., no losses due to the resistance in the transformer windings, resistor 104 is chosen to be 600 ohms so that the impedance seen by the telephone line looking into the telephone line interface circuit is 600 ohms. Capacitor 110 functions as a decoupling capacitor. Capacitor 110 essentially passes AC signals with frequencies over 10 Hz and blocks AC signals with frequencies less than 10 Hz, and, of course, blocks the DC component of the telephone line signal. This prevents any DC current from entering secondary winding 122 of coupling transformer 106, which is generally designed for linear operation without any DC current, i.e., coupling transformer 106 is a xe2x80x9cdryxe2x80x9d transformer. In the circuit of FIG. 1, the value of capacitor 110 can be 22 xcexcF and can have a voltage rating of 62 volts.
Diode bridge 114 rectifies the telephone line voltage and current applied to electronic inductor 112. Since electronic inductor 112 is implemented with transistors, which are essentially polar devices, i.e., they require a DC bias of specific polarity to operate, diode bridge 114 is added to telephone line interface circuit 100 to ensure that a positive voltage and a negative voltage are always applied to the DC positive and DC negative terminals of electronic inductor 112, respectively, regardless of the line voltage polarity present at the tip and ring terminals of telephone line interface circuit 100. This avoids the possibility that incorrect wiring of a telephone wall jack will result in a malfunction of telephone line interface circuit 100 due to a polarity mismatch.
MOV 118 functions as a voltage surge suppressor. When the voltage across the tip and ring terminals of the telephone line exceeds approximately 300 volts, MOV 118 clamps the voltage at the tip and ring terminals of the telephone line to a maximum value, thus protecting electronic inductor 112.
Relay 116 allows current flow from the telephone line if relay 116 is closed. In other words, the telephone line interface circuit is off-hook. If relay 116 is open, there is an open circuit and therefore no current flow. In other words, the telephone line interface circuit is on-hook. Relay 116 is turned off and on by means of a relay control in SSD 102 (the connection between the relay control and relay 116 is not shown in FIG. 1). One reason Relay 116 is necessary to the circuit shown in FIG. 1 is because it is required that telephone line interface circuits, such as the circuit of FIG. 1, must comply with certain requirements for on-hook maximum current flow and AC impedance. Generally, the standard requires that on-hook DC current flow be less than 10 xcexcA and that on-hook AC impedance be greater than 5 kilo ohms.
To meet these specifications, assuming a typical Central Office battery voltage of approximately 50 volts, a minimum resistance of 5 meg ohms is required between the tip and ring terminals of the telephone line interface circuit (50 Volts /10 xcexcA=5 meg ohms) when the circuit is on-hook. Relay 116 has previously been used to meet this requirement by completely disconnecting the telephone line interface circuit from the telephone line. The resistance of an open circuit is infinite and therefore there is no current flow when relay 116 is open.
In FIG. 1, electronic inductor 112 is shown in block diagram form. When relay 116 is closed (i.e., in the off-hook state) current is allowed to flow from the telephone line tip and ring terminals through line 107 and 162 and into electronic inductor 112. Electronic inductor 112 sets the DC current value for the telephone line interface circuit.
The use of a relay hookswitch in telephone line interface circuits has disadvantages. One disadvantage of using a relay is its physical size. Relays are bulky and occupy a large space. Another disadvantage is that a relay requires a relatively large amount of power to be activated. In addition, the opening of a relay generally induces undesirable high-voltage spikes across its terminals. Also, a relay must completely disconnect the telephone line interface circuit from the telephone line to meet on-hook DC resistance and AC impedance requirements, which results in total signal isolation from the telephone line in the on-hook state. Moreover, a relay is a relatively expensive device.
Solid state relays have recently been introduced which are relatively small and self-contained compared to mechanical relays. However, these solid state relays must have excellent linearity characteristics and must withstand high voltages to be employed as a hookswitch. Therefore, the use of a solid state relay as a hookswitch generally increases the cost of a telephone line interface circuit even though it reduces physical size.
Accordingly, there is a need in the art for an improved telephone line interface circuit which does not require a hookswitch relay to place the telephone line interface circuit in the on-hook and off-hook states.
The present invention is a telephone line interface circuit which does not require a hookswitch relay. The invention eliminates the need for a bulky relay, thus saving space. In addition, high-voltage spikes induced across the relay""s terminals when the relay opens are eliminated. Furthermore, the invention""s telephone line interface circuit operates using less power than the telephone line interface circuits which use relays. Moreover, the invention""s telephone line interface circuit is less expensive as compared with the telephone line interface circuits utilizing relays.
The invention uses a switch, preferably an optoisolator device, to place a telephone line interface circuit on-hook and off-hook. The optoisolator is not in series with the telephone line interface circuit with respect to the tip and ring terminals of the telephone line. In one embodiment, the optoisolator is controlled by a control signal generated by a communication device such as a modem. An output of the optoisolator is coupled to the input of a DC control circuit in a telephone line interface circuit. An output of the DC control circuit is coupled to a AC switch. When the optoisolator input control signal is activated, the optoisolator output enables voltage or current bias to the transistor, which places the telephone line interface circuit off-hook. When the optoisolator input control signal is deactivated, the optoisolator output disables voltage or current bias to the transistor, which places the telephone line interface circuit on-hook.
In one embodiment, the DC current control circuit is an electronic inductor comprising a pair of NPN transistors connected in a Darlington configuration. In one embodiment, the AC switch comprises a PNP transistor whose base is coupled to the output of the DC control circuit. In another embodiment the AC switch comprises an NPN transistor in series with the secondary winding of the coupling transformer. The base of NPN transistor is driven by the optoisolator.